what does Unlimited Update mean on my Photos on Phone 8?

It means, that Photos will use your cellular connection without any limits to update the library. This can become very expensive:

https://help.apple.com/iphone/11/#/iph961b96c4dhttps://help.apple.com/iphone/11/#/iph961b96c4d

Note: To always use cellular data to upload and download from iCloud Photo Library, go to Settings > Photos > Cellular Data, then turn on Unlimited Updates.

Where are you seeing this? What level of iOS are you using?

That’s what I was curious about, as it does let you enable it without having to have cellular data on...was hopeful since I’ve been trying to download my photo library from iCloud to my phone for 2 weeks.

Does unlimited updates do anything if “upload over cellular data” is turned off?

I have never tested this, but if you disable the use cellular data, iCloudPhoto Library should be restricted to Wi-Fi for updating.